64 PHYSICAL PROPERTIES OF MINERALS. 
the hardness is less on the basal plane of the prism, that is, on 
the cleavage surface, than it is on the sides of the prism. On 
the contrary, the terminaticn of a crystal of cyanite is harder 
than the lateral planes. ‘The degree of hardness in different 
directions may be obtained with great accuracy by means of an 
instrument called a sclerometer. 
2. TENACITY. 
The following rather indefinite terms are used with reference 
to the qualities of tenacity, malleability, and flexibility in min- 
erals : 
1. Brittle.—When a mineral breaks easily, or when parts of 
the mineral separate in powder on attempting to cut it. 
2. Malleuble-—When slices may be cut off, and these slices 
will flatten out under the hammer, as in native gold, silver, 
copper. 
3. Sectile.—When thin slices may be cut off with a knife. 
All malleable minerals are sectile. Argentite and cerargyrite 
are examples of sectile ores of silver. The former cuts nearly 
like lead and the latter nearly like wax, which it resembles. | 
Minerals are imperfectly sectile when the pieces cut off pul- 
verize easily under a hammer, or barely hold together, as sele- 
nite. . 
4. Flexible. —When the mineral will bend, and remain bent 
after the bending force is removed. Example, talc. 
5. Elastic.—When, after being bent, it will spring back to 
its original position. Example, mica. 
A liquid is said to be viscous when on pouring it the drops 
lengthen and appear ropy. Example, petroleum. 
3. SPECIFIC GRAVITY. 
The specific gravity of a mineral is its weight compared with 
that of some substance taken as a standard. For solids and 
liquids distilled water, at 60° F., is the standard ordinarily 
used ; and if a inineral weighs twice as much as water, its spe- 
cific gravity is 2; if three times itis three. It is then necessary 
to compare the weight of the mineral with the weight of sn 
equal bulk of water. The process is as follows: 
First weigh a fragment of the mineral in the ordinary way, 
